Proton shifts in gaseous arenium ions and their role in the gas-phase

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J . Am. Chem. SOC.1992, 114,6776-6784

6776

Proton Shifts in Gaseous Arenium Ions and Their Role in the Gas-Phase Aromatic Substitution by Free Me3C+ and Me3Si+ Cations Fulvio Cacace, Maria Elisa Crestoni, and Simonetta Fornarini* Contribution from the Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Universitd degli Studi “La Sapienza”, p . le A . Moro, 5, 00185 Rome, Italy. Received March 4, 1992

Abst~act: The gas-phase reaction of Me&+ with benzene and toluene has been studied at 720 Torr, in the temperature range

from 47 to 120 OC with the radiolytic technique, using suitably deuterated substrates. The results underline the kinetic role of the 1,2-H(D) shifts in the primary charged intermediate, the u complex I. When the latter originates from Me$+ attack at the para position of p-MeC6H4D,the D shift rate is relatively slow, ca. 5 X lo7 s-l at 47 “C, with an activation energy E, = 7.6 & 0.2 kcal mol-’ and a preexponential factor A = 1012.9M.4.Ions I from the Me3C+ attack on benzene or at the meta positions of toluene have lifetimes shorter than a few nanoseconds, and the 1,2-H shift, while still kinetically significant in the overall benzene reaction, is unaffected by addition of strong bases to the gas at pressures up to 8 Torr, which ensures deprotonation of I within 1 11s. Since 1,2-H(D) shifts compete with back dissociationof I, their rate influences the net alkylation rate, which accounts for the kinetic isotope effect in excess of 2 observed in the competitive tert-butylation of C6H6 and C&, the first one reported for a thermal ionic reaction in the gas phase. Analogous experiments involving the reaction of Me3Si+ with benzene and toluene show that H(D) shifts are remarkably slow from the ipso-silylated position, consistent with its high site basicity demonstrated by previous mass spectrometric and radiolytic studies.

The study of electrophilic aromatic substitution in the gas phase is of considerable interest, since it allows one to establish the intrinsic features of this important class of reactions, unperturbed by the effects of the medium so pronounced in condensed systems. Full exploitation of gas-phase approaches and meaningful kinetic correlation with solution chemistry presupposes, however, overcoming certain peculiar problems of gas-phase ion-molecule interactions. Of particular concern is the “electrostatic activation’’ occurring in the low-pressure domain, where inefficient collisional thermalization allows the energy released by the electrostatic ion-molecule interaction to remain stored in the degrees of freedom of the collision complex formed, with important kinetic consequences. First, it is impossible to define a kinetically meaningful temperature. Furthermore, the excess internal energy can allow the collision complex to overcome the activation barrier of the reaction, i.e., referring to the general outline of aromatic substitution reactions given in Scheme I, kl > k,, and the collision complex (ArH-E’) evolves into the u complex I, in all events.’ In this case the substitution displays no selectivity, since the experimentally observed relative reactivities nearly reflect relative collision rates (Scheme I). The problem can be overcome by working in a bath gas sufficiently dense to allow collisional thermalization of the (ArH.E+) complex before its conversion into I. Moreover, formation of the collision complex is less exothermic in a dense gas than under isolated-pair conditions since E+ is likely to be associated with a bath-gas molecule M, so that the energy released in the process

(M-E’)

+ ArH

-

(ArH.E+)

+M

(1)

amounts only to the difference between the binding energies of the ion E+ to ArH and to M. As a typical example, the collision frequency of a (Me$+-C6&) complex in isobutane at 40 ‘c and 760 Torr is of the order of 5 X 1Olo s-l, and more than 40% of the Me$+ ions are present as (Me3C+-i-C4Hlo)complexes.2 The pressure required to achieve thermalization of the collision complex, the “high-pressure limit” above which ion-molecule reactions obey thermal kinetics, depends of course on the specific system studied, but in general it exceeds by orders of magnitude the pressure range accessible to current mass spectrometric techniques. (1) (a) Cacace, F. Acc. Chem. Res. 1988, 21, 215. (b) Cacace, F.; Crestoni, M. E.; Di Marzio, A.; Fornarini, S. J. Phys. Chem. 1991, 95, 8731. (c) Speranza, M. Int. J. Mass Spectrom. Ion Processes, in press. (2) Sunner, J. A,; Hirao, K.; Kebarle, P. J. Phys. Chem. 1989, 93, 4010.

0002-7863/92/ 1514-6776$03.00/0

Scheme I

kC

E+ + ArH

[ArH

*

E+]

k,

E [ArH

*

E+]

k1

k. 1

QH I

ArE + BH+

R

E

R k ‘4

tI

..a.

Therefore, most of the kinetic data on thermal aromatic substitutions by gaseous cations have been obtained with the radiolytic technique, characterized by a pressure range up to several atmospheres and allowing direct evaluation of the substrate and positional selectivity of the substitution by direct isolation and characterization of the isomeric end products.’ Many of the charged electrophiles so far studied in the gas phase, e.g., the lower carbenium ions, display a very high reactivity by solution chemistry standards, reflecting the lack of a solvation shell and of a count e r i ~ n . As ~ a consequence, there is a low activation barrier to the u complex, k l > k , in Scheme I, and the reaction occurs at the encounter rate, lacking substrate and positional discrimination, e.g., alkylation by the ethyl cation in methane at atmospheric pressure is characterized by a k,o,uenc/kbcnrcnc ratio of ca. 0.8, with a meta ratio as low as 1.3 at 37.5 oC.3a The above considerations explain the quest for less reactive cations and the special interest attached to Me$+, whose relatively mild electrophilic character and the stringent steric requirements (3) (a) Cacace, F.; Cipollini, R.; Giacomello, P.; Poasagno, E. Gazz. Chim. Ital. 1974,104, 977. (b) Cacace, F.; Passagno, E. J. Am. Chem. SOC.1973, 95, 3397. (c) AttinP, M.; Cacace, F.; Ciranni, G.; Giacomello, P. J. Am. Chem. SOC.1977, 99, 2611. (d) Attini, M.; Cacace, F.; Ciranni, G.; Giacomello, P. J. Chem. SOC.,Perkin Trans. 2 1979, 891. (e) AttinH, M.; Giacomello, P. J. Am. Chem. Soc. 1979, 101,6040. (0 Aliprandi, B.;Cacace, F.; Fornarini, S. Tetrahedron 1987, 43, 2831. (g) Cerichelli, G.; Crestoni, M. E.; Fornarini, S.J. Am. Chem. SOC.1992, 114, 2002.

0 1992 American Chemical Society

Proton Shifts in Gaseous Arenium Ions endow its reaction with arenes with the required sensitivity to the effects of substituents and to structural changes of the substrate. Gas-phase aromatic tert-butylation has been the focus of extensive mass spectrometric and radiolytic s t ~ d i e s . Formation ~ of tert-butylated CT complexes from benzene is well documented by pulsed high-pressure mass spectrometry in the pressure range up to 4 Torr. Me&+ C6H6 Me3C-C6H6+ (2)

+

-+

= -22 f 2 kcal mol-' and &Toz Kebarle et al. have reported W 2 = -49 f 5 cal K-' mol-', assigning the ionic product the 4protonated tert-butylbenzene structure.s These data have been confirmed in a recent study by Stone and Stone,6 who have examined, in addition, the alkylation of toluene,

-

Me$+ C7H8 Me3C-C7H8+ (3) assigning the charged product the 4-, or 6-protonated 3-tert-b~= -29.1 f 0.3 kcal tyltoluene structure and measuring both W 3 mol-' and ASo3 = -54.6 f 0.8 cal K-' mol-'. Neither value changes appreciably in passing from C7H8 to C7D8, and the tert-butylated adduct from the latter has been found to transfer a deuteron to gaseous bases,consistent with the suggested arenium ion structure.6 Radiolytic experiments, carried out in isobutane and neopentane at atmospheric pressure, have provided pertinent kinetic informatione4 At 37.5 OC Me3C+ions display a remarkable substrate ratio of 55 f 5, discrimination, measured by a ktoluene/kknzene unusually high for a gaseous cation and even higher than in conventional Friedel-Crafts tert-butylation reactions.4a The orientation is also selective, e.g., in toluene there is no detectable ortho substitution, and the para/'/, meta ratio is ca. 35. From ratio, obtained from a the Arrhenius plot of the ktoluene/kknzenc temperature-dependence study in isobutane at 720 Torr, a 3.6 f 0.4 kcal mol-' difference between the activation energies for the tert-butylation of benzene and of toluene has been estimated.4a On the basis of these results, the present study is aimed at elucidating the detailed course of aromatic substitution by gaseous Me$+ ions, especially as regards the factors influencing the competition between back dissociation of the ipso-substituted arenium ion I and its evolution into the products. Since the competition depends to a large extent on the rapidity of the proton shifts from the ipso carbon to other ring positions, a major objective of this study is the evaluation of the rate constants of proton migrations in gaseous arenium ions, a subject of considerable intrinsic interest, currently the focus of numerous experimental and theoretical investigations.' Finally, isotopic labeling techniques, in particular the study of deuterated arenes, allow evaluation of kinetic H / D isotope effects in proton migration and of their influence on the net alkylation rate. The information obtained is by no means irrelevant since, despite the widespread use of deuterated reactants in mass spectrometry, preciously little is known on H / D kinetic isotope effects (KIEs) in gas-phase ionmolecule reactions obeying thermal kinetics, the only ones amenable to standard quantitative treatments and comparable to KIEs in solution. The study has been extended to the closely related aromatic substitution by gaseous Me3Si+ ions, which provides a useful comparison term, owing to the widely different effects of the Me3C and Me3Si groups on the basicity of the ipso carbon relative to (4) (a) Cacace, F.; Ciranni, G. J. Am. Chem. SOC.1986, 108, 887. (b) Cacace, F.; Giacomello, P. J . Am. Chem. Soc. 1973.95, 5851. (c) Giacomello, P.; Cacace, F. J. Am. Chem. SOC.1976, 98, 1823. (d) Aliprandi, B.; Cacace, F.; Cipollini, R. Radiochim. Acta 1982, 31, 107. (5) Sen Sharma, D. K.; Ikuta, S.;Kebarle, P. Can. J. Chem. 1982, 60, 2325. (6) Stone, J. M.; Stone, J. A. Int. J . Mass Spectrom. Ion Processes 1991, 109, 247. (7) (a) Kuck, D. Mass Spectrom. Rev. 1990, 9, 583. (b) Attina, M.; Cacace, F.; Ricci, A. J. Am. Chem. SOC.1991, 113, 5937. (c) Kuck, D.; Schneider, J.; Grntzmacher, H.-F. J. Chem. SOC.Perkin Tram. 2 1985, 689. (d) Mason, R.; Fernandez, M. T.; Jennings, K. R. J . Chem. SOC.Faraday Tram. 2 1987,83, 89. (e) Sordo, T.; Bertran, J.; Canadell, E. J . Chem. Soc., Perkin Tram 2 1979, 1486. (0 Here, W. J.; Pople, J. A. J. Am. Chem. SOC. 1972, 94, 6901.

J . Am. Chem. SOC.,Vol. 114, No. 17, 1992 6777 adjacent ring positions and hence on the rate of the proton shifts in ipso-tert-butylated (I, E = Me$) and ipso-trimethylsilylated (I, E = Me,Si) arenium ions.

Experimental Section Materials. i-C4HI0,CH4, and O2 were research grade gases from Matheson Co. with a stated purity in excess of 99.95 mol %. Most of the other chemicals used, including perdeuterated benzene and toluene, whose isotopic purity exceeded 99.96%, were commercial products from Aldrich Chimica S.r.1.or Fluka Chemie AG. Exceptions were 0-, m-, and p-deuteriotoluenes, which were synthesized from the corresponding bromotoluenes, preparing the Grignard reagents in carefully dried ether, followed by hydrolysis with D 2 0 . The deuterated toluenes were isolated and purified by preparative GLC, using a 4-m-long column packed with Carbowax 2OM-KOH (2%) on Supelcoport. The synthesis yielded a deuterium incorporation of 85 f 2%, 60 f 2%, and 73 f 2% into the p-, m-,and o-deuteriotoluenes, respectively. The D content was determined by CH4 chemical ionization mass spectrometry, comparing the intensities of [M + C2HS]+ ions after correction of the I3C contribution, using a Hewlett-Packard 5892A quadrupole instrument. An independent determination of the D content was obtained by 13C-NMR spectrometry. In the proton-decoupled "C-NMR spectra, the aromatic "C bearing the D atom is characterized by an isotope effect on the chemical shift and by a triplet absorption pattern with ' J l l c - ~= 24 Hz. The integrated triplet signal was checked against that of the corresponding H-bearing I3cto obtain the relative amounts of CH3C6H4Dversus CH3C6HS.The N M R spectra were recorded on a Varian XL 300 instrument with a "C probe operating at 75.43 MHz. Radiolytic Experiments. The gaseous samples were prepared using standard vacuum procedures in sealed 135" Pyrex vessels.lb The competition experiments involving C6H646D6 and CH3C6HS-CD3C6DS mixtures required a precise knowledge of the relative amounts of deuterated and undeuterated reactants. To this end, a calibrated stock mixture was prepared, from which the required amount for each experiment of the series was weighted. The irradiations were carried out in a 220 Gammacell (Nuclear Canada Ltd.) at total doses ranging from 1 X lo4 Gy to 2 X lo4 Gy at a dose rate of ca. 2 X lo4 Gy h-I. The radiolytic products mixture was extracted by freezing the vessel to 77 K and then washing its inner walls with ethyl acetate with repeated freezethaw cycles. The nature, the yields, and the D content of the radiolytic products were determined by GLC-MS, using a 50-m-long, 0.20-mm4.d. fused-silica column, coated with a 0.5-pm crosslinked methylsilicone film (Hewlett-Packard PONA column), mounted on a H P 5890 gas chromatograph in series with a H P mass selective detector model 5970B. The isotopic content of the tert-butylated and trimethylsilylated products was deduced from the ratio of the abundancies of the corresponding [M - 151' ions, the base peaks in their E1 mass spectra. Formation of these ions involves direct cleavage of a methyl radical from the Me3C(Si) group in the molecular ion, with no scrambling of hydrogens from the aromatic ring, as checked from suitably labeled compounds.

Results Table I illustrates the tert-butylation and trimethylsilylation of 0-, m-, and p-CH3C6H4D(o-D, m-D, and p-D) by Me3X+ions (X = C or Si) at ca. 1 atm. Both reactions are well characterized ionic reactions whose absolute radiochemical yields (G values) undergo the expected decrease upon addition of bases/nucleophiles, which intercept the reactant cations and/or their ionic precursors. Absolute yields are not affected by the presence of IO Torr of 02, an effective radical scavenger. The radiolytic reactions are typically run to less than 1% conversion of the starting substrete, which means that the substrate concentration and that of any additive comparable to it are effectively constant during the time lapse of the experiment. The tert-butylation reaction is the ope most thoroughly investigated, at three different temperatures in the case of p D and in the presence of varying amounts of pyridine, a base whose proton affinity (PA = 221 kcal mol-')* enables fast deprotonation of alkylated arenium ions. As previously reported," only the meta and para isomers are formed. For each isomer, the percentages of deuterated and unlabeled products are reported in Table I after correction for the incomplete deuteration of the substrate (Le., they refer to a hypothetical 100% monodeuterated toluene substrate, under the assumption that the residual KE(8) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes,J. L.; Levin, R. D.; Mallard, N. G. J . Phys. Chem. Rex Data Suppl. 1 1988, 17.

Cacace et al.

6778 J . Am. Chem. SOC.,Vol. 114, No. 17, 1992 Table I. Gas-Phase Reaction of Me3X+Ions (X = C or Si) with o-, m-,and p-CH3C6H.,D (0-D, m-D, and p D )

system composition (Torr)"

products: Me3X-substitutedtoluenes isomeric H(D) content in composition (a) each isomerb (76) meta para meta para

bulk temp reactant toluene pyridine gas ("C) ion: Me3X+ p-D, 1.8 695 47 x=c 20 80 23 (77) 45 (55) D-D. 1.7 1.2 690 47 x=c 8 92 27 (73) 82 (18) 47 x=c 9 91 p-D; 2.1 1.5 685 27 (73j 84 ( i 6 j x=c 47 7 27 (73) 93 2.4 680 p-D, 1.8 87 (13) x=c 47 8 92 695 p-D, 1.6 90 (10) 26 (74) 3.3 x=c 4.7 47 4 96 690 p-D, 2.0 27 (73) 92 (8) x=c 47 7.3 26 (74) 5 95 695 p-D, 1.7 95 (5) x=c 37 0.8 80 63 680 p-D, 0.9 60 (40) 25 (75) x=c 28 80 72 680 p-D, 1.7 71 (29) 2.2 25 (75) x=c 80 710 p-D, 1.5 22 3.8 24 (76) 78 78 (22) x=c 19 5.6 25 (75) 80 81 690 p-D, 1.2 81 (19) 120 x=c 51 49 680 p-D, 1.0 53 (47) 1.1 25 (75) x=c 120 p-D, 1.3 40 2.1 60 700 64 (36) 25 (75) x=c 120 39 3.9 61 690 p-D, 1.3 26 (74) 66 (34) x=c 120 690 p-D, 1.7 35 6.1 65 71 (29) 26 (74) 47 m-D, 1.6 x=c 9 1.2 91 700 15 (85) 28 (72) m-D, 1.2 x=c 47 8 1.9 700 27 (73) 92 13 (87) x=c 47 m-D, 1.5 6 3.1 94 700 13 (87) 30 (70) 47 m-D, 1.8 700 x=c 6 94 4.0 30 (70) 12 (88) x=c 710 47 7 m-D, 1.7 7.3 28 (72) 93 10 (90) x=c 47 0-D, 2.4 685 12 21 (79) 88 11 (89) 0-D, 2.3 1.o x=c 680 47 10 90 12 (88) 4 (96) x=c 690 47 6 94 0-D, 2.3 3.2 10 (90) 3 (97) x=c 690 47 0-D, 2.3 4.6 5 12 (88) 95 1 (99) x=c 0-D, 1.6 3.9 700 120 28 72 12 (88) 3 (97) 0-D, 1.2 x=c 7.0 680 120 25 12 (88) 75 1 (99) X = Si 1.3' 680 47 17 p-D, 1.4 0 (100) 83 98 (2) m-D, 0.9 0.8c 710 47 X = Si 20 80 40 (60)' 0 (100) "The gaseous systems contained 0,(10 Torr). The bulk gas was i-C4HI0(X = C) or a 35/1 mixture of CH4-(CH3)4Si(X = Si). bThe deuterium content in each isomer has been normalized with respect to a hypothetical 100% deuterated substrate, considering that the % fraction of undeuterated substrate would give an equal % fraction of undeuterated products. Standard deviation of data: &2%. CTriethylaminehas been used instead of pyridine.

deuterated toluene contributes to the formation of the unlabeled product proportionally to its mole fraction). In the case of mtert-butyltoluene, irrespective of the position of the label, the results obtained in the presence of pyridine are invariant with respect to changes in the temperature and the base concentration. On the other hand, the D content of the p-tert-butyltoluene from the reaction of p-D decreases markedly at higher pyridine concentrations. A less pronounced, but still significant increase of the D content is found in ptert-butyltoluene from the reaction of m-D, while nearly constant values, close to 100% D retention, are found in the product from the reaction of o-D in the presence of pyridine. The D content of p-tert-butyltoluene from p D is higher at higher temperatures, reaching 47%at 120 O C in the presence of 1.1 Torr of pyridine. The Competitive tert-butylation Of C6H6> 2. Such a lower limit for k,(H)/kz(D) ( R = H) may be compared to the grossly ap~~~

(16) Cacace, F.; Crestoni, M. E.; de Petris, G.; Fornarini, S.; Grandinetti, F. Can. J . Chem. 1988, 66, 3099.

k'k2 c m

~~~

(17) Ausloos, P.; Lias, S. G. J . Am. Chem. Soc. 1977, 99, 4198. (18) Reference 11, p 21.

Proton Shifts in Gaseous Arenium Ions

J . Am. Chem. SOC.,Vol. 114, No. 17, 1992 6783

I=

k c a l mol"

!

160

Figure 7. Energy profile for the attack of Me$+ to the meta position of toluene. The stability of the isomeric protonated m- and p-rert-butyltoluenes has been estimated assuming the additivity of tert-butyl and

methyl substituent effects.21The binding energy of the [Me3C+.PhMe] collision complex has been taken equal to that of [Me2C1+.PhMe].22 proximate k2(H)/k2(D) = 8 f 4 (R = p-Me) derived from the data of Table 111. Despite the large uncertainties affecting both sets of data, they concur in showing that a fairly large isotope effect may be attached to thermal 1,2-H shifts in arenium ions, and to our knowledge, they represent the f m t recognition of isotope effects in these processes. A related process is the aromatic hydrogen exchange in concentrated aqueous sulfuric acid, though it concerns intermolecular, rather than intramolecular proton transfer. Several data from different workers have been recently analyzed, showing relatively large isotope effects ( k H / k D= ca 5.3 f 2.1) for the deprotonation of arenium inter~nediates.'~The 1,2-H shift taking place in the k2 step implies a bent transition state, and current semiclassical theories on primary isotope effects predict a reduction of its maximum value when a bending character of the reaction coordinate is involved." However, two items should be considered. First, the observed isotope effect is not purely primary, but particularly as regards the competition of C6H6 versus C6D6, the H / D substitution involves not only the reaction center and the migrating atom but also farther positions, whose vibrational frequencies will also change. Thus, even restricting the analysis to the elementary 1,2 migration step, this will include several contributions whose relative weight is not easily predictable. Second, the range of KIEs in gas-phase reactions can conceivably be different from that observed in condensed-phase reactions, where coupling between the proton motion and the environment can be strong, e.g., a relatively large KIE ( k H / k D = 3.7 at 300 K) has been calculated recently for the intramolecular proton transfer in isolated hydrogenoxalate anion.20 The insight into the timing of 1,2-H shifts following Me3C+ ion attack on monodeuterated toluenes and the overall k H / k Dratios from the C~HSCH~/C~D&!D~ competition concur in outlining a coherent picture of the detailed course of gas-phase tert-butylation of toluene. The attack at the para position gives an arenium ion stabilized by a p-methyl group, according to a process more exothermic than the corresponding reaction at the meta position of toluene or with benzene, as shown by the energy profiles in (19) Cox,R. A. J . Phys. Org. Chem. 1991, 4, 233. (20) Truong, T. N.; McCammon, J . A. J . Am. Chem. SOC.1991, 113,

75n4..

(21) Devlin, J. L., 111; Wolf, J. F.; Taft, R. W.; Here, W. J. J . Am. Chem. SOC. 1976, 98, 1990. (22) Magnera, T. F.; Kebarle, P. In Ionic Processes in the Gas Phase; Almoster Ferreira, M. A,, Ed.: Reidel: Dordrecht, 1984; p 135.

hk

blC

\%e

I,

Figure 8. Energy profile for the attack of Me$+ to the para position of toluene. See notes to Figure 7.

figures 7 and 8. It seems reasonable to argue that the ensuing arenium ion I, (I, = I, R = p-Me) will be less prone to back dissociation, i.e., klis smaller. Its fate will then be determined 2 since neither k-2 nor k3 are significant, and by the k B [ B ] / kratio, eq 8 will hold. In agreement with these expectations k H / k Dvalues are observed to decrease in the presence of pyridine, e&, k H / k D varies from 1.3 with no bases added to 1.0 in the presence of 4.4 Torr of pyridine at 47 'c. At 120 OC, the kH/kDratios, still sensitive to the addition of pyridine, have somewhat higher values, traced to increased k-l, whereas kBundergoes the weak negative temperature effect typical of collisional rate constants. The kinetic pattern that characterizes Me3C+attack at the meta position of toluene is quantitatively different from that pertaining to its attack a t the para position. Changes in the base concentration were seen not to affect the fast H migration both at 47 OC and at 120 'C, and eq 9 then applies. Accordingly, the k H / k D ratios are unaffected by the presence of the base, and that the k H / k Dratio = 1.0 a t 47 O C is indicative of k2 >> k-l at that temperature, whereas the two constants must become comparable to 120 'C leading to kH/kD= 1.5. The Me3C+ ion attacks on benzene or a t the meta positions of toluene bear some similarity due to the lack of the activating effect of a p-methyl group, and their kYl are probably close. Therefore, since eq 9 likely holds in both cases, their different kH/kDratios point to a large difference in the k2 rate constants, either in absolute value and/or in isotopic sensitivity. It may be recalled that when R = m-Me the k2 step leads to arenium ions stabilized by two alkyl groups, as evidenced in Figure 7. An alternative explanation for the significant KIE observed in the formations of tert-butylbenzene and of m-tert-butyltoluene, the latter only at 120 OC, involves a rate-determining concerted process wherein the electrophilic attack proceeds simultaneously with the migration of H from the site of attack. This hypothesis, however, seems hardly compatible with the observed temperature dependence of k H / k D . The above discussion lends support to the idea that the Me3C+ attack at the para position of toluene is endowed with a higher degree of reversibility than the attack at the meta positions, Le., (k2/k-l)meu>> (k2/k-l)pra.The ultimate reason for this behavior lies in the fact that the isomeric 2- and 4-protonated 3-tert-b~tyltoluenes formed from the meta attack and the first ensuing H shift are the most stable species, toward which the system tends to evolve, consistent with previous results on the electrophilic tert-butylation of gaseous a r e n e ~ As . ~ a matter of fact, the isomeric composition of the products reported in Tables I and I1 shows that formation of m-tert-butyltoluene increases at the expense of the para isomer with increasing temperature and decreasing base concentration, two factors which affect the internal energy and

Cacace et al.

6184 J . Am. Chem. SOC.,Vol. 114, No. 17, 1992 the lifetime of I. The questions arise as to which kind of reversibility is operative in the k-, step or by which mechanism the p-tert-butyltoluenium m-tert-butyltoluenium isomerization takes place. At least three routes may be envisioned: (i) back dissociation of I, to the collision complex or even to the free reactants, followed by further reaction to form I, (I, = I, R = m-Me); (ii) intramolecular 1,2 migration of a tert-butyl group, I,; (iii) intermolecular transfer of Me3C+ from I, to i.e., I, a second molecule of toluene forming I,. Circumstantial, evidence favoring an isomerization via the collision complex (route i) rather than by 1,2-tert-butyl group migration (route ii) comes from the protonation-induced isomerization of o-tert-butyltoluene to both m- and p-tert-b~tyltoluenes.~~The Occurrence of route iii is suggested by a slight decrease in the extent of meta substitution when the toluene concentration is lowered (cf. the last two entries of Table 11), suggestive of a transalkylation reaction involving an arene molecule. Similar transalkylation reactions of the type Me3CC&+ C ~ H S R Me3CC6H5R+ C&j ( R = Me Or Me3C) have been suggested to occur in a CI ion source, based on the faster disappearance of Me3CC6H6+ion at increasing concentrations of C6HSR.

-

-

+

-

+

Conclusions The results provide a deeper insight into the detailed mechanism of gas-phase aromatic tert-butylation, especially as regards a sharp kinetic discrimination among its consecutive steps and the evaluation of their influence on the overall reaction rate. Under conditions ensuring compliance to thermal kinetics, gas-phase tert-butylation displays mechanistic features closely related to those of classical electrophilic substitution in condensed media, in particular, the present results provide conclusive evidence for the intermediacy and the mechanistic role of ipsesubstituted arenium ions I in the gas phase as well as in solution. The basicity of the ipso-substituted carbon relative to the adjacent ring positions is the key factor which determines the lifetime and the fate of I, and hence the course and the net rate of the alkylation. If the ipso-substituted ion I is stabilized by a suitably located activating group, e.g., in I,, its lifetime, which depends on the sum of k-, and k2, is sufficiently long (10-g-10-7 s) to allow deprotonation by bases present in the gas at the concentrations typical of radiolytic experiments (1016-10’7molecule ~ m - ~ On ) . the other hand, the lifetime of unstabilized ions, as those from benzene and I, ions from toluene, is appreciably shorter, reflecting the increase of k-I and of k2. The former increases, owing to the less favorable (23) The intermediacy of [ Me,C’.ArH] complexes has been recently inferred in diphenylalkane systems: Kuck, D.; Matthias, C. J . Am. Chem. SOC. 1992, 111, 1901.

energetics for the formation of the arenium ion I from the collision complex, which shifts to the left the equilibrium between the two adducts. The value of k2 is also expected to be larger in I, than in I, ions,since the proton shift from the ipso to an ortho position is thermodynamically favored in I, and nearly thermoneutral in I,. In the case of benzene the balance of k-I and k2 makes proton shift from the ipso position kinetically significant over the entire temperature interval investigated, 47-120 OC, whereas in the case of toluene it becomes significant only at 120 OC. Within this general reactivity pattern, this work has focused on the direct determination of the rate constant for 1,2-H(D) shift within arenium ions. The use of selectively deuterated toluenes has allowed the evaluation of the k4 and k6 rate constants, related to the H(D) shift, following Me3C+ion attack at the para position of p-MeC6H4D and m-MeC6H4D. The Arrhenius activation parameters obtained for the I1 111 D shift, E , = 7.6 f 0.2 kcal mol-’ and log A = 12.9 f 0.4, are a significant addition to the very limited set of data presently available on the rates of thermal H shifts within gaseous arenium Both the estimated k6/k4 ratio and the analysis of the overall kH/kDvalues in the tertbutylation of C6H6-C6D6mixtures concur with the conclusion that the primary kinetic isotope effect associated with 1,2-H shifts following the a-complex formation are unexpectedly large for a H migration process involving a far from linear reaction coordinate. The results concerning the reactions of Me3Si+ with benzene and toluene fit into the general scheme which identifies the relative basicity of the ipso-substituted carbon as the paramount mechanistic factor. In fact, the silylated Q complexes VI11 display hardly any tendency to undergo 1,2-H shifts, a behavior clearly related to the enhanced basicity of the ips9trimethylsilylated position of VIII. The present findings c o n f m previous conclusions regarding the mechanism of gas-phase cationic silylation and of related alkyl- or proto-desilylation reaction^.^^^^^'^ The results bear witness to the powers of the radiolytic technique, underlining the advantages derived from its time resolution, of the order of the nanosecond, from its wide pressure range, extending to the high-pressure domain where thermal kinetics apply, and from the isolation of the end products, which allows full exploitation of isotopic labeling techniques. In this connection, it can be noted that the H / D isotope effect which represents one of the most significant and mechanistically informative features of the thermal tert-butylation reaction vanishes altogether when the reaction is studied by mass spectrometric techniques.

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Acknowledgment. We are grateful to Dr. G . Cerichelli for the N M R spectra. Work was financially supported by the Minister0 dell’Universit2 e della Ricerca Scientifica e Tecnologica (MURST) and by the National Research Council (CNR).